The term “optical microsystems” is used here to mean image sensing or image display systems formed from a monolithic chip (in principle, made of silicon) on which has been etched on the one hand a dot matrix array in rows and columns and on the other hand electronic circuits controlling this matrix array; an optical image-forming structure is placed in close contact with the silicon chip, the chip and attached optical structure together forming the optical microsystem.
In the case of an image sensor, the matrix arrayed dots are photosensitive elements generating electrical signals according to the light received by the cell; the optical image-forming structure is then a system (objective lens, in principle) capable of projecting an overall image to be detected onto the matrix array of photosensitive cells formed on the monolithic chip.
Conversely, in the case of an image display device, the dots are electrooptical cells emitting, or transmitting, or reflecting light, modulating it according to an electrical signal applied to the cell; the optical image-forming structure is then a structure capable of transforming the electrical signals applied to the matrix array of electrooptical cells formed on the chip into a visible image, real or virtual.
These optical Microsystems are intended for applications in which miniaturization is essential. Examples of these include miniature optical modulators designed to project video signals, viewfinders of photographic equipment or cameras, display screens and miniature cameras on mobile phone handsets, etc.
The cost of these systems is largely due to the cost of fabrication of the integrated circuit chip. This cost in turn results on the one hand from the area of silicon used to fabricate a chip, and on the other hand from the nature and number of operations involved in fabrication. In particular, the cost is the result of the collective or noncollective nature of the operations, in the sense that certain operations are carried out collectively on a silicon wafer comprising a certain number of individual chips, whereas other operations must be carried out individually on each chip after cutting the wafer into individual chips. If certain operations cannot be carried out collectively, the cost is increased. For the collective operations, if the area of silicon used is larger for a chip, only a smaller number of chips can be placed on a wafer, so increasing the cost of the operations for each chip.
The addition to the electronic chip of an optical structure raises similar cost problems since, on the one hand, additional operations are necessarily required to produce this structure and, on the other hand, the need to add an optical system to the chip can modify and make more complex or more costly the operations required for the electronic chip itself.
One object of the invention is therefore to improve the ratio between the performance and the cost of fabrication of an optical microsystem.